1. Field of the Invention
This invention pertains generally to magnetic materials, and more particularly to a method for producing rare-earth magnets that have improved magnetic properties.
2. Description of the Background Art
Rare earth magnets are utilized in a wide variety of applications, ranging from motors, magnetic bearings, magnetic couplings, speakers, microphones, signal recording devices, instrumentation, switches, and relays to charged particle beam guidance, nuclear magnetic resonance image-forming equipment and particle accelerators.
A class of rare earth magnets, composed of a formulation of neodymium (Nd), iron (Fe) and boron (B), are the most powerful magnets available. They have high residual inductance (Br), high maximum energy product (BH.sub.max) and relatively high intrinsic coercivity (H.sub.ci). These magnets, known as the neodymium-iron-boron Rare earth magnets (Nd.sub.2 Fe.sub.14 B), provide the best magnetic properties in terms of high energy product. With its excellent magnetic properties, abundant raw material for its manufacture, and a relatively low manufacturing cost, use of the Nd.sub.2 Fe.sub.14 B rare earth magnets in applications requiring high magnetic properties are becoming almost universal. Rare earth-containing magnets are typically produced by crushing a rare earth-containing alloy into magnetic powder, forming and molding the magnetic powder into magnetic bodies (known as green compacts), sintering and heat-treating the green compacts.
Due to it's active chemical nature, however, neodymium is easily oxidized to form Nd.sub.2 O.sub.3. Research has shown that the oxidation of neodymium in the magnet is seriously destructive to the properties of rare earth-containing magnets. The oxygen content of neodymium-iron-boron rare earth magnets is the key factor affecting the magnet's properties. Nd.sub.2 O.sub.3, which is formed through the reaction of neodymium with O.sub.2, has anti-magnetic characteristics and detrimental to the overall magnetic properties of Nd.sub.2 Fe.sub.14 B rare earth magnets. Thus, reducing the oxygen content in the magnet effectively improves the magnetic properties of neodymium-iron-boron rare earth magnets because the Nd.sub.2 O.sub.3 content is correspondingly reduced. Reducing the percentage of Nd.sub.2 O.sub.3 in the major phase of Nd.sub.2 Fe.sub.14 B will also result in a reduction of volume and weight of the magnet, thus allowing for miniaturization at a very economical cost without sacrificing performance.
Accordingly, there is a need for a cost-effective method for manufacturing rare earth containing magnets that consistently reduces and/or controls the ability of oxygen-containing sources to react with the magnetic powder during the manufacture of the rare earth containing magnet. The present invention satisfies this need, as well as others, and generally overcomes the deficiencies found in the background art.